ABSTRACT

Basic Information

Abstract Number: 2590 - 4
Author Name: Markus B Raschke - University of Colorado
Session Title: Biological TERS: Instrumentation Development and Applications
Event Type: Symposia
Event Title: Vibrational Coupling and Dynamics from Low-Temperature Tip-Enhanced Raman Spectroscopy

Presider Name:Igor Lednev
Affiliation:University at Albany, SUNY

Date: Thursday, March 12, 2015
Start Time: 02:45 PM (Slot #4)
Location: 239

Abstract Content

Intra- and intermolecular coupling and dynamics determines chemical reaction, charge transfer, biological, and catalytic activity of molecules. Spectroscopic access has continued to challenge vibrational photon-echo and its multidimensional spectroscopy implementations to probe the homogeneous system response. In contrast, tip-enhanced Raman spectroscopy (TERS) can intrinsically probe at the homogeneous sample size limit by virtue of its nano-meter spatial resolution and single molecule sensitivity. We perform systematic TERS measurements at cryogenic and variable temperature of dilute malachite green monolayer physisorbed on a template stripped gold surface. The experiments reveal details of the molecular dynamics over a wide range from few 100’s fs intramolecular dephasing to seconds for low temperature structural reorientations. Line narrowing, splitting, and the appearance of slow mode hopping with decreasing temperature shows that the inhomogeneous broadening at room temperature is characterized by high-frequency fluctuations of molecular orientation and/or structure. The temperature dependence allows for the separation of a temperature independent dephasing process from an Arrhenius-type activated process with a few 100 cm[sup]-1[/sup] activation energy indicative of vibrational stretch and combination band coupling to low energy intramolecular bending and torsional modes. Corresponding coupling constant and lifetime of the low frequency modes are consistent with literature values from related photon echo experiments. In the homogeneous sample size limit we observe spectral fluctuations that can be interpreted as discrete orientational changes of the molecules on the surface. The correlation of spectral fluctuations of coupled normal modes provide additional evidence of a single molecule response.